Spark plug having a substantially columnar electrode tip welded to a component thereof

ABSTRACT

An object of the invention is to restrain separation of an electrode tip from a ground electrode or from a center electrode. A spark plug satisfies a condition 0.8≦A1/A2≦1.9, where, as viewed on a section which contains a center axis (O) of an electrode tip, A1 [Hv] is hardness of the electrode tip as measured outside a circle having a radius of 0.2 mm, with a boundary point (PA) on a surface of the spark plug between a fusion zone and the electrode tip serving as the center of the circle, and A2 [Hv] is hardness of the electrode tip as measured within the circle having a radius of 0.2 mm.

FIELD OF THE INVENTION

The present invention relates to a spark plug having electrode tips.

BACKGROUND OF THE INVENTION

Conventionally, in order to improve erosion resistance of an electrodeportion of a spark plug, an electrode tip formed from a noble metal,such as platinum, iridium, ruthenium, or rhodium, or from a noble metalalloy is provided on a distal end portion of a ground electrode and on afront end portion of a center electrode (see Japanese Patent ApplicationLaid-Open (kokai) No. 2006-32185, hereinafter referred to as PatentDocument 1). Such electrode tips are usually laser-welded to the centerelectrode and to the ground electrode.

Generally, metals used to form the center electrode and the groundelectrode differ from those used to form the electrode tips.Accordingly, the center electrode and the ground electrode differ inthermal expansion coefficient from the electrode tips. Thus, in acertain welded condition in which the electrode tip is welded to thecenter electrode or to the ground electrode, the electrode tip mayseparate from the center electrode or from the ground electrode, forexample, in a heat load test in which heating and cooling are repeated,or in the course of operation of an internal combustion engine.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In view of the above problem, an object of the present invention is torestrain separation of an electrode tip from a ground electrode or froma center electrode.

Means for Solving the Problems

The present invention has been conceived for solving, at leastpartially, the above problem and can be embodied in the following modesor application examples.

APPLICATION EXAMPLE 1

A spark plug in which a substantially columnar electrode tip is weldedto a distal end portion of a ground electrode, and a fusion zone isformed through the welding at a boundary portion between the groundelectrode and the electrode tip, the spark plug satisfying a condition0.8≦A1/A2≦1.9, where, as viewed on a section which contains a centeraxis of the electrode tip, A1 [Hv] is hardness of the electrode tip asmeasured outside a circle having a radius of 0.2 mm, with a boundarypoint on a surface of the spark plug between the fusion zone and theelectrode tip serving as the center of the circle, and A2 [Hv] ishardness of the electrode tip as measured within the circle having aradius of 0.2 mm.

APPLICATION EXAMPLE 2

A spark plug according to application example 1, satisfying a condition0.7≦B2/B1≦2.5, where, as viewed on the section, B1 is hardness [Hv] ofthe ground electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the ground electrode serving as the center of thecircle, and B2 is hardness [Hv] of the ground electrode as measuredwithin the circle having a radius of 0.2 mm.

APPLICATION EXAMPLE 3

A spark plug according to application example 1 or 2, wherein the groundelectrode has a pedestal portion protruding from a distal end portionthereof, and the electrode tip is placed on the pedestal portion and isjoined to the pedestal portion by means of a boundary portion betweenthe pedestal portion and the electrode tip being subjected to welding.

APPLICATION EXAMPLE 4

A spark plug according to any one of application examples 1 to 3,wherein the ground electrode has a linear expansion coefficient greaterthan that of the electrode tip.

APPLICATION EXAMPLE 5

A spark plug in which a substantially columnar electrode tip is weldedto a top surface of a convex intermediate tip having a flange portion ata bottom thereof to thereby form a composite tip having a fusion zoneformed through the welding at a boundary portion between theintermediate tip and the electrode tip, and the composite tip is joinedto a distal end portion of the ground electrode via the flange portion,the spark plug satisfying a condition 0.8≦A1/A2≦1.9, where, as viewed ona section which contains a center axis of the electrode tip, A1 [Hv] ishardness of the electrode tip as measured outside a circle having aradius of 0.2 mm, with a boundary point on a surface of the spark plugbetween the fusion zone and the electrode tip serving as the center ofthe circle, and A2 [Hv] is hardness of the electrode tip as measuredwithin the circle having a radius of 0.2 mm.

APPLICATION EXAMPLE 6

A spark plug according to application example 5, satisfying a condition0.5≦B2/B1≦2.5, where, as viewed on the section, B1 is hardness [Hv] ofthe ground electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the intermediate tip serving as the center of thecircle, and B2 is hardness [Hv] of the intermediate tip as measuredwithin the circle having a radius of 0.2 mm.

APPLICATION EXAMPLE 7

A spark plug according to application example 5 or 6, wherein theintermediate tip has a linear expansion coefficient greater than that ofthe electrode tip.

APPLICATION EXAMPLE 8

A spark plug according to any one of application examples 1 to 7,satisfying a condition 60°≦θ≦150°, where, as viewed on a section whichcontains the center axis of the electrode tip and provides a maximumwidth of the fusion zone as measured along the center axis, θ is anangle between the center axis and a perpendicular bisector of a linesegment connecting an upper end and a lower end of the fusion zone on asurface of the spark plug.

APPLICATION EXAMPLE 9

A spark plug in which a substantially columnar electrode tip is weldedto a front end portion of a center electrode, and a fusion zone isformed through the welding at a boundary portion between the centerelectrode and the electrode tip, the spark plug satisfying a condition0.8≦A1/A2≦1.9, where, as viewed on a section which contains a centeraxis of the electrode tip, A1 [Hv] is hardness of the electrode tip asmeasured outside a circle having a radius of 0.2 mm, with a boundarypoint on a surface of the spark plug between the fusion zone and theelectrode tip serving as the center of the circle, and A2 [Hv] ishardness of the electrode tip as measured within the circle having aradius of 0.2 mm.

APPLICATION EXAMPLE 10

A spark plug according to application example 9, satisfying a condition0.7 B2/B1≦2.3, where, as viewed on the section, B1 is hardness [Hv] ofthe center electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the center electrode serving as the center of thecircle, and B2 is hardness [Hv] of the center electrode as measuredwithin the circle having a radius of 0.2 mm.

APPLICATION EXAMPLE 11

A spark plug according to application example 9 or 10, wherein thecenter electrode has a linear expansion coefficient greater than that ofthe electrode tip.

APPLICATION EXAMPLE 12

A spark plug in which a substantially columnar electrode tip is weldedto a top surface of a convex intermediate tip having a flange portion ata bottom thereof to thereby form a composite tip having a fusion zoneformed through the welding at a boundary portion between theintermediate tip and the electrode tip, and the composite tip is joinedto a front end portion of the center electrode via the flange portion,the spark plug satisfying a condition 0.8≦A1/A2≦1.9, where, as viewed ona section which contains a center axis of the electrode tip, A1 [Hv] ishardness of the electrode tip as measured outside a circle having aradius of 0.2 mm, with a boundary point on a surface of the spark plugbetween the fusion zone and the electrode tip serving as the center ofthe circle, and A2 [Hv] is hardness of the electrode tip as measuredwithin the circle having a radius of 0.2 mm.

APPLICATION EXAMPLE 13

A spark plug according to application example 12, satisfying a condition0.7≦B2/B1≦2.3, where, as viewed on the section, B1 is hardness [Hv] ofthe center electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the intermediate tip serving as the center of thecircle, and B2 is hardness [Hv] of the intermediate tip as measuredwithin the circle having a radius of 0.2 mm.

APPLICATION EXAMPLE 14

A spark plug according to application example 12 or 13, wherein theintermediate tip has a linear expansion coefficient greater than that ofthe electrode tip.

APPLICATION EXAMPLE 15

A spark plug according to any one of application examples 9 to 14,satisfying a condition 30°≦θ≦90°, where, as viewed on a section whichcontains the center axis of the electrode tip and provides a maximumwidth of the fusion zone as measured along the center axis, θ is anangle between the center axis and a perpendicular bisector of a linesegment connecting an upper end and a lower end of the fusion zone on asurface of the spark plug.

APPLICATION EXAMPLE 16

A spark plug according to any one of application examples 1 to 15,wherein the electrode tip has a width D measured along a directionperpendicular to the center axis of the electrode tip and satisfying acondition 0.4 D (mm) 1.2

APPLICATION EXAMPLE 17

A spark plug according to any one of application examples 1 to 16,wherein the electrode tip is a member which contains at least one ofplatinum, iridium, ruthenium, and rhodium.

Effects of the Invention

According to the spark plug of application example 1, in a structure inwhich the electrode tip is joined directly to the ground electrode, apreferred relation can be established between hardness A2 of theelectrode tip as measured at a portion located in the vicinity of theboundary between the fusion zone and the electrode tip and influenced bywelding heat, and hardness A1 of the electrode tip as measured at aportion not influenced by welding. Thus, there can be restrainedseparation, caused by thermal stress, of the electrode tip welded to theground electrode at a boundary portion between the electrode tip and thefusion zone.

According to the spark plug of application example 2, in a structure inwhich the electrode tip is joined directly to the ground electrode, apreferred relation can be established between hardness B2 of the groundelectrode as measured at a portion located in the vicinity of theboundary between the fusion zone and the ground electrode and influencedby welding heat, and hardness B1 of the ground electrode as measured ata portion not influenced by welding. Thus, there can be restrainedseparation, caused by thermal stress, of the electrode tip together withthe fusion zone from the ground electrode.

According to the spark plug of application example 3, in a structure inwhich the electrode tip is joined to a pedestal portion protruding froma distal end portion of the ground electrode, separation of theelectrode tip from the pedestal portion can be restrained.

According to the spark plug of application example 4, even in the casewhere the linear expansion coefficient of the ground electrode isgreater than that of the electrode tip, separation of the electrode tipcaused by thermal stress can be restrained through satisfaction of theabove-mentioned hardness conditions.

According to the spark plug of application example 5, in a structure inwhich the electrode tip is joined to the ground electrode via theintermediate tip, a preferred relation can be established betweenhardness A2 of the electrode tip as measured at a portion located in thevicinity of the boundary between the fusion zone and the electrode tipand influenced by welding heat, and hardness A1 of the electrode tip asmeasured at a portion not influenced by welding. Thus, there can berestrained separation, caused by thermal stress, of the electrode tipwelded to the intermediate tip at a boundary portion between theelectrode tip and the fusion zone.

According to the spark plug of application example 6, in a structure inwhich the electrode tip is joined to the ground electrode via theintermediate tip, a preferred relation can be established betweenhardness B2 of the intermediate tip as measured at a portion located inthe vicinity of the boundary between the fusion zone and theintermediate tip and influenced by welding heat, and hardness B1 of theground electrode as measured at a portion not influenced by welding.Thus, there can be restrained separation, caused by thermal stress, ofthe electrode tip together with the fusion zone from the intermediatetip.

According to the spark plug of application example 7, even in the casewhere the linear expansion coefficient of the intermediate tip isgreater than that of the electrode tip, separation of the electrode tipcaused by thermal stress can be restrained through satisfaction of theabove-mentioned hardness conditions.

According to the spark plug of application example 8, since theelectrode tip can be welded to the ground electrode at a preferredwelding angle, an increase in stress caused by thermal deformation canbe restrained. Thus, separation of the electrode tip can be restrainedmore effectively.

According to the spark plug of application example 9, in a structure inwhich the electrode tip is joined directly to the center electrode, apreferred relation can be established between hardness A2 of theelectrode tip as measured at a portion located in the vicinity of theboundary between the fusion zone and the electrode tip and influenced bywelding heat, and hardness A1 of the electrode tip as measured at aportion not influenced by welding. Thus, there can be restrainedseparation, caused by thermal stress, of the electrode tip welded to afront end portion of the center electrode at a boundary portion betweenthe electrode tip and the fusion zone.

According to the spark plug of application example 10, in a structure inwhich the electrode tip is joined directly to the center electrode, apreferred relation can be established between hardness B2 of the centerelectrode as measured at a portion located in the vicinity of theboundary between the fusion zone and the center electrode and influencedby welding heat, and hardness B1 of the center electrode as measured ata portion not influenced by welding. Thus, there can be restrainedseparation, caused by thermal stress, of the electrode tip together withthe fusion zone from the center electrode.

According to the spark plug of application example 11, even in the casewhere the linear expansion coefficient of the center electrode isgreater than that of the electrode tip, separation of the electrode tipcaused by thermal stress can be restrained through satisfaction of theabove-mentioned hardness conditions.

According to the spark plug of application example 12, in a structure inwhich the electrode tip is joined to the center electrode via theintermediate tip, a preferred relation can be established betweenhardness A2 of the electrode tip as measured at a portion located in thevicinity of the boundary between the fusion zone and the electrode tipand influenced by welding heat, and hardness A1 of the electrode tip asmeasured at a portion not influenced by welding. Thus, there can berestrained separation, caused by thermal stress, of the electrode tipwelded to a front end portion of the center electrode via theintermediate tip at a boundary portion between the electrode tip and thefusion zone.

According to the spark plug of application example 13, in a structure inwhich the electrode tip is joined to the center electrode via theintermediate tip, a preferred relation can be established betweenhardness B2 of the intermediate tip as measured at a portion located inthe vicinity of the boundary between the fusion zone and theintermediate tip and influenced by welding heat, and hardness B1 of thecenter electrode as measured at a portion not influenced by welding.Thus, there can be restrained separation, caused by thermal stress, ofthe electrode tip together with the fusion zone from the intermediatetip.

According to the spark plug of application example 14, even in the casewhere the linear expansion coefficient of the intermediate tip isgreater than that of the electrode tip, separation of the electrode tipcaused by thermal stress can be restrained through satisfaction of theabove-mentioned hardness conditions.

According to the spark plug of application example 15, since theelectrode tip can be welded to a front end portion of the centerelectrode at a preferred welding angle, an increase in stress caused bythermal deformation can be restrained. Thus, separation of the electrodetip can be restrained more effectively.

According to the spark plug of application example 16, the width of theelectrode tip required for laser welding can be ensured, and the effectof restraining separation through satisfaction of the above-mentionedhardness conditions and angle conditions can be favorably yielded.

According to the spark plug of application example 17, erosionresistance of the ground electrode or the center electrode can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially sectional view showing a spark plug 100 accordingto a first embodiment of the present invention.

FIG. 2 is an enlarged view showing a distal end portion of a groundelectrode 30 of the first embodiment.

FIG. 3 is a sectional view showing a distal end portion of the groundelectrode 30 of the first embodiment.

FIG. 4 is an enlarged view showing a distal end portion of the groundelectrode 30 of a second embodiment.

FIG. 5 is a sectional view showing a distal end portion of the groundelectrode 30 of the second embodiment.

FIG. 6 is an enlarged view showing a distal end portion of the groundelectrode 30 of a third embodiment.

FIG. 7 is a sectional view showing a distal end portion of the groundelectrode 30 of the third embodiment.

FIG. 8 is an enlarged view showing a front end portion of a centerelectrode 20 of a fourth embodiment.

FIG. 9 is a sectional view showing a front end portion of the centerelectrode 20 of the fourth embodiment.

FIG. 10 is an enlarged view showing a front end portion of the centerelectrode 20 of a fifth embodiment.

FIG. 11 is a sectional view showing a front end portion of the centerelectrode 20 of the fifth embodiment.

FIG. 12 is a view showing imaginary circles with a boundary point PAserving as the centers thereof on the section of an electrode tip 70.

FIG. 13 is a table showing the results of measurement of hardness in theimaginary circles.

FIG. 14 is a graph in which hardness ratios in the imaginary circles areplotted.

FIG. 15 is a table showing the results of judgment on separation in thefirst to third embodiments.

FIG. 16 is a table showing the results of judgment on separation in thefourth and fifth embodiments.

FIG. 17 is a view showing the definition of a separation rate used as acriterion for judgment on separation.

FIG. 18 is a sectional view of the ground electrode 30 at a weldingangle of 90° or greater.

FIG. 19 is a table showing the results of a separation evaluation testconducted on samples of the second embodiment having different weldingangles θ.

FIG. 20 is a table showing the results of a separation evaluation testconducted on samples of the fourth embodiment having different weldingangles θ.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will next be described withreference to the drawings in the following order:

A. First embodiment (a ground electrode having an electrode tip, type1);

B. Second embodiment (a ground electrode having an electrode tip, type2);

C. Third embodiment (a ground electrode having an electrode tip, type3);

D. Fourth embodiment (a center electrode having an electrode tip, type1);

E. Fifth embodiment (a center electrode having an electrode tip, type2);

F. Evaluation tests; and

G Conclusion.

A. FIRST EMBODIMENT A Ground Electrode Having an Electrode Tip, Type 1

FIG. 1 is a partially sectional view of a spark plug 100 according to afirst embodiment of the present invention. The spark plug 100 includes aceramic insulator 10, a center electrode 20, a ground electrode 30, ametal terminal 40, and a metallic shell 50. The center electrode 20 hasa rodlike shape, projects from one end of the ceramic insulator 10, andis electrically connected to the metal terminal 40 provided at the otherend of the ceramic insulator 10, through the interior of the ceramicinsulator 10. The outer circumference of the center electrode 20 is heldby the ceramic insulator 10, and the outer circumference of the ceramicinsulator 10 is held by the metallic shell 50 at a position located awayfrom the metal terminal 40. The ground electrode 30 is electricallyconnected to the metallic shell 50 and forms a spark gap, across whichsparks are generated, in cooperation with the front end of the centerelectrode 20. The spark plug 100 is mounted, via the metallic shell 50,to a mounting threaded hole 201 formed in an engine head 200 of aninternal combustion engine. When a high voltage of 20,000 to 30,000volts is applied to the metal terminal 40, sparks are generated acrossthe spark gap formed between the center electrode 20 and the groundelectrode 30.

The ceramic insulator 10 is an insulator formed through firing of aceramic material, such as alumina. The ceramic insulator 10 is a tubularmember having an axial bore 12 formed at the center thereof and adaptedto accommodate the center electrode 20 and the metal terminal 40. Theceramic insulator 10 has a center trunk portion 19 formed at the axialcenter thereof and having a large outside diameter. The ceramicinsulator 10 has a rear trunk portion 18 located on a side toward themetal terminal 40 with respect to the center trunk portion 19 andadapted to electrically insulate the metal terminal 40 and the metallicshell 50 from each other. The ceramic insulator 10 has a front trunkportion 17 located on a side toward the center electrode 20 with respectto the center trunk portion 19 and having an outside diameter smallerthan that of the rear trunk portion 18. The ceramic insulator 10 has aleg portion 13 located frontward of the front trunk portion 17 andhaving an outside diameter which is smaller than that of the front trunkportion 17 and reduces toward the front end of the center electrode 20.

The metallic shell 50 is a cylindrical metal member which holds theceramic insulator 10 therein while surrounding a region of the ceramicinsulator 10 extending from a subportion of the rear trunk portion 18 tothe leg portion 13. In the present embodiment, the metallic shell 50 isformed from low-carbon steel. The metallic shell 50 includes a toolengagement portion 51, a mounting threaded portion 52, and a sealportion. 54. The tool engagement portion 51 of the metallic shell 50allows a tool (not shown) for mounting the spark plug 100 to the enginehead 200 to be fitted thereto. The mounting threaded portion 52 of themetallic shell 50 has a thread to be threadingly engaged with themounting threaded hole 201 of the engine head 200. The seal portion 54of the metallic shell 50 is a flange-like portion formed at the base ofthe mounting threaded portion 52. An annular gasket 5 formed by foldinga sheet is fitted between the seal portion 54 and the engine head 200. Afront end surface 57 of the metallic shell 50 has an annular shape. Atthe center of the front end surface 57, the center electrode 20 projectsfrom the leg portion 13 of the ceramic insulator 10.

The center electrode 20 is a rodlike electrode having a structure inwhich a core 25 superior in thermal conductivity to an electrode basemetal 21 is embedded within the electrode base metal 21 having aclosed-bottomed tubular shape. In the present embodiment, the electrodebase metal 21 is formed from a nickel alloy which contains nickel as amain component. The core 25 is formed from copper or from an alloy whichcontains copper as a main component. The center electrode 20 is insertedinto the axial bore 12 of the ceramic insulator 10 in such a manner thatthe front end of the electrode base metal 21 projects from the axialbore 12 of the ceramic insulator 10. The center electrode 20 iselectrically connected to the metal terminal 40 via a ceramic resistor 3and a seal body 4.

The ground electrode 30 is joined to the front end surface 57 of themetallic shell 50 and is bent in a direction intersecting with thedirection of an axis O of the center electrode 20 and such that theinner surface of a distal end portion thereof faces the front end of thecenter electrode 20. In the present embodiment, the ground electrode 30is formed from a nickel alloy, such as INCONEL, which contains nickel asa main component.

FIG. 2 is a view showing, on an enlarged scale, a distal end portion ofthe ground electrode 30. As shown in FIG. 2, a circular columnarelectrode tip 70 is joined by laser welding to a distal end portion ofthe ground electrode 30 at a position which faces the center electrode20. In FIG. 2, an appearance before laser welding is shown at the leftof the axis O, and a section taken after laser welding is shown at theright. The electrode tip 70 is a member provided for improvingresistance to spark-induced erosion of the ground electrode 30. Theelectrode tip 70 is formed from a material which contains a noble metalof high melting point as a main component. The electrode tip 70 isformed from, for example, platinum (Pt), iridium (Ir), ruthenium (Ru),or rhodium (Rh) or from an alloy thereof. In the present embodiment, theelectrode tip 70 is formed from a Pt—Ir alloy.

As shown in FIG. 2, the ground electrode 30 and the electrode tip 70 arelaser-welded together through radiation of a laser beam to the boundarybetween the ground electrode 30 and the electrode tip 70 from adirection oblique to the boundary along the full circle. As a result ofsuch laser welding, a boundary portion between the ground electrode 30and the electrode tip 70 is melted, thereby forming a fusion zone 23.The ground electrode 30 and the electrode tip 70 have different linearexpansion coefficients. The linear expansion coefficient of the groundelectrode 30 is greater than that of the electrode tip 70. Specifically,the ground electrode 30 of the present embodiment is formed from INCONEL601 (trade name) and has a linear expansion coefficient of 17.8. Theelectrode tip 70 is formed from a Pt—Ir alloy and has a linear expansioncoefficient of 10.0. In the present embodiment and the followingembodiments, the linear expansion coefficient is a value as measured at1,000° C. and in [×10⁻⁶].

FIG. 3 is a view showing the section of a distal end portion of theground electrode 30 which contains the center axis O of the electrodetip 70. In FIG. 3, a tip width D is of the electrode tip 70 as measuredin a direction orthogonal to the center axis O and can be 0.4 mm to 1.2mm inclusive. The present embodiment employs the electrode tip 70 havinga tip width D of 0.6 mm.

The spark plug 100 of the first embodiment is formed in such a manner asto satisfy the condition 0.8≦A1/A2≦1.9, where, as viewed on a sectionwhich contains the center axis O of the electrode tip 70, A1 [Hv] is thehardness of the electrode tip 70 as measured outside a circle having aradius of 0.2 mm, with a boundary point PA on a surface of the sparkplug 100 between the fusion zone 23 and the electrode tip 70 serving asthe center of the circle, and A2 [Hv] is the hardness of the electrodetip 70 as measured within the circle having a radius of 0.2 mm. Thiscondition is hereinafter referred to as the “tip hardness condition.” Inthe structure of the ground electrode 30 of the first embodiment,through satisfaction of the above-mentioned tip hardness condition,separation of the electrode tip 70 from the fusion zone 23 can berestrained. Hardness [Hv] is Vickers hardness as measured by thehardness measuring method specified in Japanese Industrial Standards(JIS) “Z 2244” under the following conditions: a test force of 1.961 Nand a holding time of 15 sec.

Also, the spark plug 100 of the first embodiment is formed in such amanner as to satisfy the condition 0.7≦B2/B1≦2.5, where, as viewed onthe section which contains the center axis O of the electrode tip 70, B1[Hv] is the hardness of the ground electrode 30 as measured outside acircle having a radius of 0.2 mm, with a boundary point PB on a surfaceof the spark plug 100 between the fusion zone 23 and the groundelectrode 30 serving as the center of the circle, and B2 [Hv] is thehardness of the ground electrode 30 as measured within the circle havinga radius of 0.2 mm This condition is hereinafter referred to as the“ground-electrode hardness condition.” In the structure of the groundelectrode 30 of the first embodiment, through satisfaction of theabove-mentioned ground-electrode hardness condition, separation of theelectrode tip 70 together with the fusion zone 23 from the groundelectrode 30 can be restrained.

Furthermore, in the first embodiment, the electrode tip 70 and theground electrode 30 are laser-welded together in such a manner as tosatisfy the condition 60°≦θ≦150°, where, as viewed on a section whichcontains the center axis O of the electrode tip 70 and provides themaximum vertical width of the fusion zone 23, θ (hereinafter referred toas the “welding angle θ”) is an angle between the center axis O and aperpendicular bisector of a line segment connecting the boundary pointPA and the boundary point PB. This condition is hereinafter referred toas the “welding angle condition.” In the structure of the groundelectrode 30 of the first embodiment, through satisfaction of theabove-mentioned welding angle condition, separation of the electrode tip70 from the ground electrode 30 can be more effectively restrained.

The tip hardness condition, the ground-electrode hardness condition, andthe welding angle condition mentioned above for the first embodiment aredetermined on the basis of the results of various evaluation tests to bementioned later. The contents of the evaluation tests will be describedin detail later.

B. SECOND EMBODIMENT A Ground Electrode Having an Electrode Tip, Type 2

FIG. 4 is a view showing, on an enlarged scale, a distal end portion ofthe ground electrode 30 of a spark plug 100 b of the second embodiment.As shown in FIG. 4, in the second embodiment, a pedestal portion 31protrudes from a side of a distal end portion of the ground electrode 30which faces the center electrode 20, and the electrode tip 70 islaser-welded onto the pedestal portion 31. The pedestal portion 31 canbe formed by subjecting the base metal of the ground electrode 30 to,for example, press forming or cutting. Also, in the second embodiment,the linear expansion coefficient of the ground electrode 30 is greaterthan that of the electrode tip 70. Specifically, the ground electrode 30is formed from INCONEL 601 and has a linear expansion coefficient of17.8. The electrode tip 70 is formed from a Pt—Ni alloy and has a linearexpansion coefficient of 13.4.

FIG. 5 is a view showing the section of a distal end portion of theground electrode 30 which contains the center axis O of the electrodetip 70. Also, in the present embodiment, similar to the firstembodiment, the tip width D can be 0.4 mm to 1.2 mm inclusive. Thepresent embodiment employs the electrode tip 70 having a tip width D of0.6 mm.

Similar to the first embodiment, the spark plug 100 b of the secondembodiment is formed in such a manner as to satisfy the tip hardnesscondition 0.8≦A1/A2≦1.9, where, as viewed on a section which containsthe center axis O of the electrode tip 70, A1 [Hv] is the hardness ofthe electrode tip 70 as measured outside a circle having a radius of 0.2mm, with the boundary point PA on a surface of the spark plug 100 bbetween the fusion zone 23 and the electrode tip 70 serving as thecenter of the circle, and A2 [Hv] is the hardness of the electrode tip70 as measured within the circle having a radius of 0.2 mm. In thestructure of the ground electrode 30 of the second embodiment, throughsatisfaction of the above-mentioned tip hardness condition, separationof the electrode tip 70 from the fusion zone 23 can be restrained.

Similar to the first embodiment, the spark plug 100 b of the secondembodiment is formed in such a manner as to satisfy the ground-electrodehardness condition 0.7≦B2/B1≦2.5, where, as viewed on the section whichcontains the center axis O of the electrode tip 70, B1 [Hv] is thehardness of the ground electrode 30 as measured outside a circle havinga radius of 0.2 mm, with the boundary point PB on a surface of the sparkplug 100 b between the fusion zone 23 and the ground electrode 30serving as the center of the circle, and B2 [Hv] is the hardness of theground electrode 30 as measured within the circle having a radius of 0.2mm. In the structure of the ground electrode 30 of the secondembodiment, through satisfaction of the above-mentioned ground-electrodehardness condition, separation of the electrode tip 70 together with thefusion zone 23 from the pedestal portion 31 of the ground electrode 30can be restrained.

Furthermore, similar to the first embodiment, in the second embodiment,the electrode tip 70 and the pedestal portion 31 of the ground electrode30 are laser-welded together in such a manner as to satisfy the weldingangle condition 60°≦θ≦150°. In the structure of the ground electrode 30of the second embodiment, through satisfaction of the above-mentionedwelding angle condition, separation of the electrode tip 70 from thepedestal portion 31 of the ground electrode 30 can be more effectivelyrestrained.

The tip hardness condition, the ground-electrode hardness condition, andthe welding angle condition mentioned above for the second embodimentare determined on the basis of the results of various evaluation teststo be mentioned later. The contents of the evaluation tests will bedescribed in detail later.

C. THIRD EMBODIMENT A Ground Electrode Having an Electrode Tip, Type 3

FIG. 6 is a view showing, on an enlarged scale, a distal end portion ofthe ground electrode 30 of a spark plug 100 c of the third embodiment.As shown in FIG. 6, in the third embodiment, the electrode tip 70 islaser-welded to the top surface of a convex intermediate tip 33 having aflange portion 32 at the bottom thereof to thereby form a composite tip34, and the composite tip 34 is resistance-welded to a distal endportion of the ground electrode 30 via the bottom flange portion 32. Theintermediate tip 33 can be formed from, for example, the same materialas that used to form the ground electrode 30. The linear expansioncoefficient of the intermediate tip 33 is greater than that of theelectrode tip 70. Specifically, the intermediate tip 33 is formed fromINCONEL 601 and has a linear expansion coefficient of 17.8. Theelectrode tip 70 is formed from a Pt—Ni alloy and has a linear expansioncoefficient of 15.0.

FIG. 7 is a view showing the section of a distal end portion of theground electrode 30 which contains the center axis O of the electrodetip 70. Also, in the present embodiment, similar to the firstembodiment, etc., the tip width D can be 0.4 mm to 1.2 mm inclusive. Thepresent embodiment employs the electrode tip 70 having a tip width D of0.6 mm.

Similar to the first embodiment and the second embodiment, the sparkplug 100 c of the third embodiment is formed in such a manner as tosatisfy the tip hardness condition 0.8≦A1/A2≦1.9, where, as viewed on asection which contains the center axis O of the electrode tip 70, A1[Hv] is the hardness of the electrode tip 70 as measured outside acircle having a radius of 0.2 mm, with the boundary point PA on asurface of the spark plug 100 c between the fusion zone 23 and theelectrode tip 70 serving as the center of the circle, and A2 [Hv] is thehardness of the electrode tip 70 as measured within the circle having aradius of 0.2 mm. In the structure of the ground electrode 30 of thethird embodiment, through satisfaction of the above-mentioned tiphardness condition, separation of the electrode tip 70 from the fusionzone 23 can be restrained.

The spark plug 100 c of the third embodiment is formed in such a manneras to satisfy the ground-electrode hardness condition 0.5≦B2/B1≦2.5,where, as viewed on the section which contains the center axis O of theelectrode tip 70, B1 [Hv] is the hardness of the ground electrode 30 asmeasured outside a circle having a radius of 0.2 mm, with the boundarypoint PB on a surface of the spark plug 100 c between the fusion zone 23and the intermediate tip 33 serving as the center of the circle, and B2[Hv] is the hardness of the intermediate tip 33 as measured within thecircle having a radius of 0.2 mm. In the structure of the groundelectrode 30 of the third embodiment, through satisfaction of theabove-mentioned ground-electrode hardness condition, separation of theelectrode tip 70 together with the fusion zone 23 from the intermediatetip 33 can be restrained.

Furthermore, similar to the first embodiment and the second embodiment,in the third embodiment, the electrode tip 70 and the intermediate tip33 are laser-welded together in such a manner as to satisfy the weldingangle condition 60°≦θ≦150°. In the structure of the ground electrode 30of the third embodiment, through satisfaction of the above-mentionedwelding angle condition, separation of the electrode tip 70 from theintermediate tip 33 can be more effectively restrained.

The tip hardness condition, the ground-electrode hardness condition, andthe welding angle condition mentioned above for the third embodiment aredetermined on the basis of the results of various evaluation tests to bementioned later. The contents of the evaluation tests will be describedin detail later.

D. FOURTH EMBODIMENT A Center Electrode Having an Electrode Tip, Type 1

FIG. 8 is a view showing, on an enlarged scale, a front end portion ofthe center electrode 20 of a spark plug 100 d of the fourth embodiment.FIG. 8 shows the front end portion of the center electrode 20 in a statein which the spark plug 100 shown in FIG. 1 is turned by 180°. As shownin FIG. 8, in the fourth embodiment, a circular columnar electrode tip71 is joined to the center of the front end surface of the centerelectrode 20, rather than to the ground electrode 30. The centerelectrode 20 and the electrode tip 70 are laser-welded together throughradiation of a laser beam to the boundary between the center electrode20 and the electrode tip 71 along the full circle. The linear expansioncoefficient of the center electrode 20 is greater than that of theelectrode tip 71. Specifically, the center electrode 20 is formed fromINCONEL 600 and has a linear expansion coefficient of 16.4. Theelectrode tip 71 is formed from an Ir—Pt alloy and has a linearexpansion coefficient of 8.9.

FIG. 9 is a view showing the section of a front end portion of thecenter electrode 20 which contains the center axis O of the electrodetip 71. In FIG. 9, the tip width D is of the electrode tip 71 asmeasured in a direction orthogonal to the center axis O and can be 0.4mm to 1.2 mm inclusive. The present embodiment employs the electrode tip71 having a tip width D of 0.6 mm.

Similar to the above-described embodiments, the spark plug 100 d of thefourth embodiment is formed in such a manner as to satisfy the tiphardness condition 0.8≦A1/A2≦1.9, where, as viewed on a section whichcontains the center axis O of the electrode tip 71, A1 [Hv] is thehardness of the electrode tip 71 as measured outside a circle having aradius of 0.2 mm, with the boundary point PA on a surface of the sparkplug 100 d between a fusion zone 24 and the electrode tip 71 serving asthe center of the circle, and A2 [Hv] is the hardness of the electrodetip 71 as measured within the circle having a radius of 0.2 mm. In thestructure of the center electrode 20 of the fourth embodiment, throughsatisfaction of the above-mentioned tip hardness condition, separationof the electrode tip 71 from the fusion zone 24 can be restrained.

The spark plug 100 d of the fourth embodiment is formed in such a manneras to satisfy the condition 0.7≦B2/B1≦2.3, where, as viewed on thesection which contains the center axis O of the electrode tip 70, B1[Hv] is the hardness of the center electrode 20 as measured outside acircle having a radius of 0.2 mm, with the boundary point PB on asurface of the spark plug 100 d between the fusion zone 24 and thecenter electrode 20 serving as the center of the circle, and B2 [Hv] isthe hardness of the center electrode 20 as measured within the circlehaving a radius of 0.2 mm. This condition is hereinafter referred to asthe “center-electrode hardness condition.” In the structure of thecenter electrode 20 of the fourth embodiment, through satisfaction ofthe above-mentioned center-electrode hardness condition, separation ofthe electrode tip 71 together with the fusion zone 23 from the centerelectrode 20 can be restrained.

Furthermore, in the fourth embodiment, the electrode tip 71 and thecenter electrode 20 are laser-welded together in such a manner as tosatisfy the welding-angle condition 30°≦θ≦90°, where, as viewed on asection which contains the center axis O of the electrode tip 71 andprovides the maximum vertical width of the fusion zone 23, θ (weldingangle θ) is an angle between the center axis O and a perpendicularbisector of a line segment connecting the boundary point PA and theboundary point PB. In the structure of the center electrode 20 of thefourth embodiment, through satisfaction of the above-mentioned weldingangle condition, separation of the electrode tip 71 from the centerelectrode 20 can be more effectively restrained.

The tip hardness condition, the ground-electrode hardness condition, andthe welding angle condition mentioned above for the fourth embodimentare determined on the basis of the results of various evaluation teststo be mentioned later. The contents of the evaluation tests will bedescribed in detail later.

E. FIFTH EMBODIMENT A Center Electrode Having an Electrode Tip, Type 2

FIG. 10 is a view showing, on an enlarged scale, a front end portion ofthe center electrode 20 of a spark plug 100 e of the fifth embodiment.As shown in FIG. 10, the composite tip 34 composed of the intermediatetip 33 and the electrode tip 71 is resistance-welded to the front endsurface of the center electrode 20 of the spark plug 100 e of the fifthembodiment. The structure of the composite tip 34 is similar to that ofthe third embodiment. The linear expansion coefficient of theintermediate tip 33 is greater than that of the electrode tip 71.Specifically, the intermediate tip 33 is formed from the same materialas that used to form the center electrode 20; i.e., INCONEL 600, and hasa linear expansion coefficient of 16.4. The electrode tip 71 is formedfrom an Ir—Pt alloy and has a linear expansion coefficient of 8.9.

FIG. 11 is a view showing the section of a front end portion of thecenter electrode 20 which contains the center axis O of the electrodetip 71. Also, in the present embodiment, similar to the fourthembodiment, the tip width D can be 0.4 mm to 1.2 mm inclusive. Thepresent embodiment employs the electrode tip 71 having a tip width D of0.6 mm.

Similar to the above-described embodiments, the spark plug 100 e of thefifth embodiment is formed in such a manner as to satisfy the tiphardness condition 0.8≦A1/A2≦1.9, where, as viewed on a section whichcontains the center axis O of the electrode tip 71, A1 [Hy] is thehardness of the electrode tip 71 as measured outside a circle having aradius of 0.2 mm, with the boundary point PA on a surface of the sparkplug 100 e between the fusion zone 24 and the electrode tip 71 servingas the center of the circle, and A2 [Hv] is the hardness of theelectrode tip 71 as measured within the circle having a radius of 0.2mm. In the structure of the center electrode 20 of the fifth embodiment,through satisfaction of the above-mentioned tip hardness condition,separation of the electrode tip 71 from the fusion zone 24 can berestrained.

The spark plug 100 e of the fifth embodiment is formed in such a manneras to satisfy the center-electrode hardness condition 0.7≦B2/B1≦2.3,where, as viewed on the section which contains the center axis O of theelectrode tip 71, B1 [Hv] is the hardness of the center electrode 20 asmeasured outside a circle having a radius of 0.2 mm, with the boundarypoint PB on a surface of the spark plug 100 e between the fusion zone 24and the intermediate tip 33 serving as the center of the circle, and B2[Hv] is the hardness of the intermediate tip 33 as measured within thecircle having a radius of 0.2 mm. In the structure of the groundelectrode 30 of the fifth embodiment, through satisfaction of theabove-mentioned center-electrode hardness condition, separation of theelectrode tip 70 together with the fusion zone 23 from the intermediatetip 33 can be restrained.

Furthermore, similar to the fourth embodiment, in the spark plug 100 eof the fifth embodiment, the electrode tip 71 and the intermediate tip33 are laser-welded together in such a manner as to satisfy the weldingangle condition 30°≦θ≦90°. In the structure of the center electrode 20of the fifth embodiment, through satisfaction of the above-mentionedwelding angle condition, separation of the electrode tip 71 from theintermediate tip 33 can be more effectively restrained.

F. EVALUATION TESTS

F-1. Grounds for Focusing on a Circle Having a Radius of 0.2 mm with aBoundary Point Serving as the Center of the Circle

Grounds for the tip hardness condition, the ground-electrode hardnesscondition, the center-electrode hardness condition, and the weldingangle condition specified in the above-described embodiments will nextbe described on the basis of the results of evaluation tests. First,regarding the tip hardness condition, the ground-electrode hardnesscondition, and the center-electrode hardness condition, there isdescribed grounds for focusing on hardness of material as measuredoutside and within a circle having a radius of 0.2 mm, with the boundarypoint PA or PB serving as the center of the circle, the boundary pointsPA and PB being located at the upper end and the lower end,respectively, of the fusion zone.

FIG. 12 shows imaginary circles drawn on a section which contains thecenter axis O of the electrode tip 70. The imaginary circles areconcentric circles whose center is located at the boundary point PA andwhose radii increase 0.1 mm by 0.1 mm from the innermost circle. Thehardness of the electrode tip 70 was measured at measuring points A, B,C, and D, which are midpoints between the imaginary circles and arelocated on a bisector of the angle between the side surface of theelectrode tip 70 and the boundary line between the fusion zone 23 andthe electrode tip 70. The electrode tip 70 formed from a Pt alloy wasused as a sample for the measurement. The measuring point “E” appearingin FIG. 12 is located on the center axis O of the electrode tip 70 andsufficiently away from the fusion zone. Hardness measured at the point Eis the hardness of the electrode tip 70 itself.

FIG. 13 is a table showing the results of measurement of hardness in theimaginary circles. FIG. 13 shows hardnesses measured at the measuringpoints as well as ratios of the hardnesses measured at the measuringpoints to the hardness of the tip itself (hereinafter, referred to asthe “hardness ratio(s)”). FIG. 14 is a graph in which hardness ratios inthe imaginary circles are plotted. As shown in FIGS. 13 and 14, thehardness and the hardness ratio of the electrode tip 70 graduallyincrease with distance from the boundary point PA. At an outside pointC; i.e., outside the imaginary circle having a radius of 0.2 mm, thehardness and the hardness ratio sharply increase, and the hardness ratioconverges to “1.” That is, a change in hardness caused by the influenceof heat of laser welding substantially settles within a circle having aradius of 0.2 mm with the end point of the fusion zone serving as thecenter of the circle, and hardness is almost unsusceptible to heat oflaser welding outside the circle having a radius of 0.2 mm. Thus, in theexperiments to be described below, there was studied the influence, onthe separation of the electrode tip 70, of the difference in hardness(hardness ratio) between a region influenced by heat of laser welding(within a circle having a radius of 0.2 mm with the boundary pointserving as the center of the circle) and a region free from theinfluence of heat of laser welding (outside a circle having a radius of0.2 mm with the boundary point serving as the center of the circle).Thus, the optimum ranges of the hardness conditions were determined.

F-2. Grounds for the Hardness Conditions

Next, grounds for the above-mentioned hardness conditions are described.Samples of the spark plugs according to the embodiments described abovewere prepared, six or seven samples each for the embodiments. Thesamples were measured for hardnesses of the relevant members atmeasuring points within and outside a circle having a radius of 0.2 mmwith the end point of the fusion zone serving as the center of thecircle. FIGS. 15 and 16 show the results of the measurement. Thehardness measuring points A1, A2, B1, and B2 are shown in FIG. 3 for thefirst embodiment; in FIG. 5 for the second embodiment; in FIG. 7 for thethird embodiment; in FIG. 8 for the fourth embodiment; and in FIG. 10for the fifth embodiment.

FIGS. 15 and 16 show the results of measurement of hardness at themeasuring points; the hardness ratio (A2/A1) with respect to theboundary point PA; the hardness ratio (B1/B2) with respect to theboundary point PB; and judgment on separation indicative of whether ornot separation has occurred, and represented by “Good” or “Failure.”“Good” indicates judgment that separation has not occurred. “Failure”indicates judgment that separation has occurred.

Whether or not separation has occurred was judged by the followingmethod. First, by use of a burner, each of the samples is heated at anelectrode tip and its vicinity to a temperature of 1,000° C. for twominutes; subsequently, the samples are exposed to the atmosphere andcooled for one minute. A series of the operations is taken as one testcycle. The samples are subjected to 1,000 test cycles. Subsequently, thesamples are sectioned so as to yield a section which contains the centeraxis O. FIG. 17 shows the thus-obtained section of the electrode tip aswell as the definition of a separation rate used as a criterion forjudgment on separation.

Then, as viewed on the section of each of the electrode tips, cracksextending from the opposite side surfaces of the electrode tip along theboundary lines between the electrode tip and the fusion zones aremeasured for their horizontal widths T1 and T2. The percentage of thetotal of the crack widths to the total of horizontal widths S1 and S2 ofthe fusion zones is obtained. The thus-obtained percentage ishereinafter referred to as the “separation rate.” When the separationrate is 50% or greater, judgment is made that separation has occurred.When the separation rate is less than 50%, judgment is made thatseparation has not occurred. A separation rate of 50% is used as acriterion for judgment for the following reason: when the separationrate is less than 50%, the electrode tip practically does not come to bedetached from the ground electrode 30 or from the center electrode 20.

Among the samples of the first embodiment shown in FIG. 15, the sampleswhich are judged to be “Good” with respect to separation have the rangeof the hardness ratio A1/A2 associated with the boundary point PA andthe range of the hardness ratio B2/B1 associated with the boundary pointPB as follows: 0.80≦A1/A2≦1.90, and 0.70≦B2/B1≦2.50.

Among the samples of the second embodiment, the samples which are judgedto be “Good” with respect to separation have the range of the hardnessratio A1/A2 and the range of the hardness ratio B2/B1 as follows:0.78≦A1/A2≦1.92, and 0.62≦B2/B1≦2.51.

Among the samples of the third embodiment, the samples which are judgedto be “Good” with respect to separation have the range of the hardnessratio A1/A2 and the range of the hardness ratio B2/B1 as follows:0.82≦A1/A2≦1.91, and 0.50≦B2/B1≦2.50.

From the above results of judgment, for the first embodiment and thesecond embodiment, as mentioned above, the range of the hardness ratioA1/A2 serving as the tip hardness condition has been determined to be0.8≦A1/A2≦1.9, and the range of the hardness ratio B2/B1 serving as theground-electrode hardness condition has been determined to be0.7≦B2/B1≦2.5.

For the third embodiment, similar to the first and second embodiments,the range of the hardness ratio A1/A2 serving as the tip hardnesscondition has been determined to be 0.8≦A1/A2≦1.9, and the range of thehardness ratio B2/B1 serving as the ground-electrode hardness conditionhas been determined to be 0.5≦B2/B1≦2.5.

Among the samples of the fourth embodiment shown in FIG. 16, the sampleswhich are judged to be “Good” with respect to separation have the rangeof the hardness ratio A1/A2 and the range of the hardness ratio B2/B1 asfollows: 0.80≦A1/A2≦1.90, and 0.70≦B2/B1≦2.32.

Among the samples of the fifth embodiment, the samples which are judgedto be “Good” with respect to separation have the range of the hardnessratio A1/A2 and the range of the hardness ratio B2/B1 as follows:0.80≦A1/A2≦1.91, and 0.70≦B2/B1≦2.33.

From the above results of judgment, for the fourth embodiment and thefifth embodiment, the range of the hardness ratio A1/A2 serving as thetip hardness condition has been determined to be 0.8≦A1/A2≦1.9, and therange of the hardness ratio B2/B1 serving as the center-electrodehardness condition has been determined to be 0.7≦B2/B1≦2.3.

Generally, when the electrode tip and the base metal (the groundelectrode, the intermediate tip, or the center electrode) arelaser-welded together, after the welding, residual stress is generatedin the vicinity of the boundary between the electrode tip and the fusionzone. Thus, in view of prevention of separation of the electrode tip atincreased temperature, it is conceivably desirable to set the hardnessA1 in the vicinity of the boundary between the electrode tip and thefusion zone higher than the hardness A2 of the electrode tip itself.However, when the hardness A1 is excessively high, cracking is generatedin the boundary of the fusion zone immediately after welding. From sucha point of view, in the above-mentioned evaluation tests, an upper limitof the hardness ratio A1/A2 of 1.9 has been obtained. However, in theabove-mentioned evaluation tests, a lower limit of the hardness ratioA1/A2 of 0.8 has been obtained, revealing that, even when the hardnessA2 is higher than the hardness A1, detachment of the electrode tip canbe practically restrained.

Next, the relation between the fusion zone and the base metal will beconsidered. In the case where the hardness B2 in the vicinity of theboundary between the fusion zone and the base metal is lower than B1, atincreased temperature, a higher effect of compressive stress isattained. Thus, the fusion zone is conceivably unlikely to separate fromthe base metal. From such a point of view, in the above-mentionedevaluation tests, a lower limit of the hardness ratio B2/B1 of 0.7 or0.5 has been obtained. Meanwhile, when the hardness B2 in the vicinityof the boundary between the fusion zone and the base metal isexcessively high, cracking is generated in the boundary immediatelyafter welding. The above-mentioned evaluation tests have revealed that,when the upper limit of the hardness ratio B2/B1 is 2.5 or 2.3, a largecrack is practically not generated, so that detachment of the electrodetip can be restrained.

F-3. Grounds for the Welding Angle (θ) Condition

Finally, grounds for the range of the welding angle θ are described.Seven samples of the spark plug according to the second embodiment wereprepared while the welding angle θ was varied from 43° to 163°. Thesamples were subjected to a separation evaluation test based on theseparation rate. In this test, the welding angle θ was adjusted by meansof changing the diameter of the pedestal portion 31 formed on the groundelectrode 30 while the electrode tip 70 having a tip width D of 0.8 mmwas used. FIG. 5 shows the section of the ground electrode 30 at awelding angle θ of less than 90°. When the welding angle becomes 90° orgreater, as shown in FIG. 18, the width of the pedestal portion 31becomes narrower than the tip width D.

FIG. 19 shows the results of the separation evaluation test conducted onthe samples of the second embodiment having different welding angles θ.FIG. 19 shows, in addition to the welding angle θ, hardness at themeasuring points B1 and B2, the hardness ratio B2/B1, and the result ofjudgment on separation. As shown in FIG. 19, the samples having awelding angle θ of 60° to 150° received a judgment of “Good” onseparation. Thus, the welding angle condition for the first to thirdembodiments in which the electrode tip 70 is provided on the groundelectrode has been determined to be 60°≦θ≦150° as mentioned above.Because of easy adjustment of the welding angle, the present testemployed the samples of the second embodiment for evaluation. However,this evaluation method can also be applied to the first and thirdembodiments.

Seven samples of the spark plug according to the fourth embodiment wereprepared while the welding angle θ was varied from 28° to 127°. Thesamples were subjected to a separation evaluation test based on theseparation rate. In this test, the welding angle θ was adjusted by meansof changing the diameter of a front end portion of the center electrode20 while the electrode tip 71 having a tip width D of 0.6 mm was used.

FIG. 20 shows the results of the separation evaluation test conducted onthe samples of the fourth embodiment having different welding angles θ.As shown in FIG. 20, the samples having a welding angle θ of 30° to 90°received a judgment of “Good” on separation. Thus, the welding anglecondition for the fourth and fifth embodiments in which the electrodetip 71 is provided on the center electrode has been determined to be30°≦θ≦90° as mentioned above. Because of easy adjustment of the weldingangle, the present test employed the samples of the fourth embodimentfor evaluation. However, this evaluation method can also be applied tothe fifth embodiment.

G. CONCLUSION

According to the spark plugs of the embodiments described above, evenwhen the electrode tip and the base metal (the ground electrode, theintermediate tip, or the center electrode) having different linearexpansion coefficients are laser-welded together, a good relation can beestablished between the hardness in the vicinity of the fusion zone andthe hardness of the base metal. Thus, there can be restrained separationor detachment of the electrode tip from the base metal caused by thegeneration of thermal stresses in the members, for example, in thecourse of operation of an internal combustion engine. As a result,erosion resistance of the spark plug can be improved; furthermore, theoccurrence of misfire caused by an increase in gap can be restrained.Also, in the embodiments described above, not only hardness in thevicinity of the fusion zone but also the welding angle θ is specified,thereby restraining an increase in stress associated with thermaldeformation. Therefore, separation or detachment of the electrode tipcan be more effectively restrained.

While the present invention has been described with reference to thevarious embodiments, the present invention is not limited thereto, butmay be embodied in various other configurations without departing fromthe gist of the invention. For example, the electrode tip may be joinedto one of the center electrode 20 and the ground electrode 30 or to bothof the center electrode 20 and the ground electrode 30.

DESCRIPTION OF REFERENCE NUMERALS

-   3: ceramic resistor;-   4: seal body;-   5: gasket;-   10: ceramic insulator;-   12: axial bore;-   13: leg portion;-   17: front trunk portion;-   18: rear trunk portion;-   19: center trunk portion;-   20: center electrode;-   21: electrode base metal;-   22: electrode-base-metal pedestal;-   23, 24: fusion zone;-   25: core;-   30: ground electrode;-   31: pedestal portion;-   32: flange portion;-   33: intermediate tip;-   34: composite tip;-   40: metal terminal;-   50: metallic shell;-   51: tool engagement portion;-   52: mounting threaded portion;-   54: seal portion;-   70, 71: electrode tip;-   100, 100 b, 100 c, 100 d, 100 e: spark plug;-   200: engine head; and-   201: mounting threaded hole.

Having described the invention, the following is claimed:
 1. A sparkplug in which a substantially columnar electrode tip is welded to adistal end portion of a ground electrode, and a fusion zone is formedthrough the welding at a boundary portion between the ground electrodeand the electrode tip, the spark plug satisfying a condition0.8≦A1/A2≦1.9, where, as viewed on a section which contains a centeraxis of the electrode tip, A1 [Hv] is hardness of the electrode tip asmeasured outside a circle having a radius of 0.2 mm, with a boundarypoint on a surface of the spark plug between the fusion zone and theelectrode tip serving as the center of the circle, and A2 [Hv] ishardness of the electrode tip as measured within the circle having aradius of 0.2 mm.
 2. A spark plug according to claim 1, satisfying acondition0.7≦B2/B1≦2.5, where, as viewed on the section, B1 is hardness [Hv] ofthe ground electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the ground electrode serving as the center of thecircle, and B2 is hardness [Hv] of the ground electrode as measuredwithin the circle having a radius of 0.2 mm.
 3. A spark plug accordingto claim 1, wherein the ground electrode has a pedestal portionprotruding from a distal end portion thereof, and the electrode tip isplaced on the pedestal portion and is joined to the pedestal portion bymeans of a boundary portion between the pedestal portion and theelectrode tip being subjected to welding.
 4. A spark plug according toclaim 1, wherein the ground electrode has a linear expansion coefficientgreater than that of the electrode tip.
 5. A spark plug in which asubstantially columnar electrode tip is welded to a top surface of aconvex intermediate tip having a flange portion at a bottom thereof tothereby form a composite tip having a fusion zone formed through thewelding at a boundary portion between the intermediate tip and theelectrode tip, and the composite tip is joined to a distal end portionof the ground electrode via the flange portion, the spark plugsatisfying a condition0.8≦A1/A2≦1.9, where, as viewed on a section which contains a centeraxis of the electrode tip, A1 [Hv] is hardness of the electrode tip asmeasured outside a circle having a radius of 0.2 mm, with a boundarypoint on a surface of the spark plug between the fusion zone and theelectrode tip serving as the center of the circle, and A2 [Hv] ishardness of the electrode tip as measured within the circle having aradius of 0.2 mm.
 6. A spark plug according to claim 5, satisfying acondition0.5≦B2/B1≦2.5, where, as viewed on the section, B1 is hardness [Hv] ofthe ground electrode as measured outside a circle having a radius of 0.2mm with, a boundary point on a surface of the spark plug between thefusion zone and the intermediate tip serving as the center of thecircle, and B2 is hardness [Hv] of the intermediate tip as measuredwithin the circle having a radius of 0.2 mm.
 7. A spark plug accordingto claim 5, wherein the intermediate tip has a linear expansioncoefficient greater than that of the electrode tip.
 8. A spark plugaccording to claim 1, satisfying a condition60°≦θ≦150°, where, as viewed on a section which contains the center axisof the electrode tip and provides a maximum width of the fusion zone asmeasured along the center axis, θ is an angle between the center axisand a perpendicular bisector of a line segment connecting an upper endand a lower end of the fusion zone on a surface of the spark plug.
 9. Aspark plug in which a substantially columnar electrode tip is welded toa front end portion of a center electrode, and a fusion zone is formedthrough the welding at a boundary portion between the center electrodeand the electrode tip, the spark plug satisfying a condition0.8≦A1/A2≦1.9, where, as viewed on a section which contains a centeraxis of the electrode tip, A1 [Hv] is hardness of the electrode tip asmeasured outside a circle having a radius of 0.2 mm, with a boundarypoint on a surface of the spark plug between the fusion zone and theelectrode tip serving as the center of the circle, and A2 [Hv] ishardness of the electrode tip as measured within the circle having aradius of 0.2 mm.
 10. A spark plug according to claim 9, satisfying acondition0.7≦B2/B1≦2.3, where, as viewed on the section, B1 is hardness [Hv] ofthe center electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the center electrode serving as the center of thecircle, and B2 is hardness [Hv] of the center electrode as measuredwithin the circle having a radius of 0.2 mm.
 11. A spark plug accordingto claim 9, wherein the center electrode has a linear expansioncoefficient greater than that of the electrode tip.
 12. A spark plug inwhich a substantially columnar electrode tip is welded to a top surfaceof a convex intermediate tip having a flange portion at a bottom thereofto thereby form a composite tip having a fusion zone formed through thewelding at a boundary portion between the intermediate tip and theelectrode tip, and the composite tip is joined to a front end portion ofthe center electrode via the flange portion, the spark plug satisfying acondition0.8≦A1/A2≦1.9, where, as viewed on a section which contains a centeraxis of the electrode tip, A1 [Hv] is hardness of the electrode tip asmeasured outside a circle having a radius of 0.2 mm, with a boundarypoint on a surface of the spark plug between the fusion zone and theelectrode tip serving as the center of the circle, and A2 [Hv] ishardness of the electrode tip as measured within the circle having aradius of 0.2 mm.
 13. A spark plug according to claim 12, satisfying acondition0.7≦B2/B1≦2.3, where, as viewed on the section, B1 is hardness [Hv] ofthe center electrode as measured outside a circle having a radius of 0.2mm, with a boundary point on a surface of the spark plug between thefusion zone and the intermediate tip serving as the center of thecircle, and B2 is hardness [Hv] of the intermediate tip as measuredwithin the circle having a radius of 0.2 mm.
 14. A spark plug accordingto claim 12, wherein the intermediate tip has a linear expansioncoefficient greater than that of the electrode tip.
 15. A spark plugaccording to claim 9, satisfying a condition30°≦θ≦90°, where, as viewed on a section which contains the center axisof the electrode tip and provides a maximum width of the fusion zone asmeasured along the center axis, θ is an angle between the center axisand a perpendicular bisector of a line segment connecting an upper endand a lower end of the fusion zone on a surface of the spark plug.
 16. Aspark plug according to claim 1, wherein the electrode tip has a width Dmeasured along a direction perpendicular to the center axis of theelectrode tip and satisfying a condition0.4≦D(mm)≦1.2.
 17. A spark plug according to claim 1, wherein theelectrode tip is a member which contains at least one of platinum,iridium, ruthenium, and rhodium.
 18. A spark plug according to claim 5,satisfying a condition60°≦θ≦150°, where, as viewed on a section which contains the center axisof the electrode tip and provides a maximum width of the fusion zone asmeasured along the center axis, θ is an angle between the center axisand a perpendicular bisector of a line segment connecting an upper endand a lower end of the fusion zone on a surface of the spark plug.
 19. Aspark plug according to claim 12, satisfying a condition30°≦θ≦90°, where, as viewed on a section which contains the center axisof the electrode tip and provides a maximum width of the fusion zone asmeasured along the center axis, θ is an angle between the center axisand a perpendicular bisector of a line segment connecting an upper endand a lower end of the fusion zone on a surface of the spark plug.
 20. Aspark plug according to claim 5, wherein the electrode tip has a width Dmeasured along a direction perpendicular to the center axis of theelectrode tip and satisfying a condition0.4≦D(mm)≦1.2.
 21. A spark plug according to claim 9, wherein theelectrode tip has a width D measured along a direction perpendicular tothe center axis of the electrode tip and satisfying a condition0.4≦D(mm)≦1.2.
 22. A spark plug according to claim 12, wherein theelectrode tip has a width D measured along a direction perpendicular tothe center axis of the electrode tip and satisfying a condition0.4≦D(mm)≦1.2.
 23. A spark plug according to claim 5, wherein theelectrode tip is a member which contains at least one of platinum,iridium, ruthenium, and rhodium.
 24. A spark plug according to claim 9,wherein the electrode tip is a member which contains at least one ofplatinum, iridium, ruthenium, and rhodium.
 25. A spark plug according toclaim 12, wherein the electrode tip is a member which contains at leastone of platinum, iridium, ruthenium, and rhodium.